1. Field of the Invention
The present invention relates to polymer compositions comprising a modified nanoparticle dispersed in a polymer and methods of preparing the same.
2. Description of the Related Art
Polymer compositions have been used to replace glass compositions in a variety of different applications, including optical devices such as lenses. The specific requirements of the application determine the type of polymer chosen. For example, in some applications, it may be desirable to provide a polymer composition having improved physical properties, such as mechanical strength, heat resistance, or toughness. In addition, processiblity, moldability, and cost may also need to be considered.
For optical devices, improved optical properties such as refractive index, transparency or clarity, and optical uniformity may also be important, and polymer compositions have these improved properties have been prepared. For example, U.S. Pat. No. 4,990,653 describes the preparation of a sulfur-containing oligomer composition which can be polymerized to form a cured product having a refractive index of 1.60 or higher along with excellent transparency and optical uniformity. In addition, U.S. Pat. No. 5,683,628 describes a photochromic resin composition comprising a specific class of di(meth)acrylate compounds which are described as having excellent heat resistance, mechanical strength, adhesion properties, moldability, and light resistance. These can be used, for example, as an antireflection film on a lens. The refractive index of the composition can be controlled by varying the amount of specific di(meth)acrylate compound and can be 1.54 or higher. However, with the ever-increasing demands for improved optical performance, along with the need for improved physical properties, processability and reduced cost, the use of these types of tailored polymers is limited.
Several types of metal oxides are known to have high refractive indexes. For example, in general, titanium dioxide (titania), zirconium oxide (zirconia), cerium oxide (ceria), tantalum oxide, niobium oxide, zinc oxide, barium titanate, and strontium titanate, have a refractive index that is greater than or equal to about 1.7. However, metal oxides may be difficult to disperse well in a polymeric system, and poor dispersions typically have poor optical properties. In order to provide improved dispersibility in various matrices, including solvents and polymer systems, modified metal oxides have been prepared, and the type of modification used depends on the specific system. For example, silane coupling agents having functionality capable of reacting with silica and with a rubber matrix have been used to prepare modified silica particles for use in tire applications. Modifications to other metal oxides, including those with high refractive indexes, are also known. However, the modification made to the metal oxide would be expected to greatly decrease the material's refractive index, thereby reducing the effectiveness of the modified metal oxide for an optical application. Furthermore, modification of nanoparticle dispersions of metal oxides (those having a particle size of less than 50 nm) often results in an undesirable increase in particle size, with subsequent loss in optical clarity. Maintaining particle size typically requires either the use of stabilizing agents, such as dispersants, which often have a negative effect on physical and optical properties, or mechanical methods of particle size reduction. For example, U.S. Patent Publication No. 2007/0036962 describes a method of preparing a nanoparticle-resin composite, which includes a step of modifying a dispersion of a metal oxide with an organic compound. The resulting modified metal oxide is then isolated by precipitation and reaggregation and subsequently dispersed in the polymer using mechanical dispersion techniques. Such multiple step methods are often cumbersome and inefficient, as well as costly and labor intensive. The resulting composite would also be expected to have undesirable optical properties, including haze and high scattering.
Thus, there is a need in the industry to provide polymer compositions for optical devices which have good overall performance, including mechanical properties and process ability, along with improved optical properties, especially high refractive index and transparency. Furthermore, there is a need for a method for preparing polymer compositions comprising a nanoparticle dispersion of a metal oxide without loss of the nanoparticle size of the metal oxide.